Carbon and hydrogen isotopic composition of sterols in natural marine brown and red macroalgae and associated shellfish

Carbon (delta C-13) and hydrogen isotopic compositions (delta D) of sterols in natural marine brown (Sargassum filicinum and Undaria pinnatifida), red macroalgae (Binghamia californica and Gelidium japonica), and in shellfish (Binghamia californica, Haliotis discus and Omphalius pfeifferi) feeding on the brown algae have been investigated as a first attempt to understand the isotopic compositions of sterols in natural algae and heterotrophs. Brown algae have 24-methylcholesta-5,24(28)-dien3 beta-ol with delta C-13 value of -19.1 +/- 0.1 parts per thousand and delta D value of -298 +/- 4 parts per thousand, and 24-ethylcholesta-5,24(28)E-dien-3 beta-ol with delta C-13 value of -20.3 +/- 0.2 parts per thousand and delta D value of -301 +/- 1 parts per thousand. Red algae have cholest-5-en-3 beta-ol with delta C-13 value of -20.9 +/- 3.0 parts per thousand and delta D value of -289 +/- 0 parts per thousand. The delta C-13 and delta D values of these sterols in algae are consistent with those of algal sterols found in marine sediments, suggesting that the isotopic compositions of algal sterols are well preserved in sediments with little change due to heterotrophic effects. The three shellfish have a wide diversity of sterol types but most sterols, except cholest-5-en-3 beta-ol, have quite similar isotope compositions (delta C-13 value of -19.5 +/- 0.4 parts per thousand and delta D value of -296 +/- 5 parts per thousand) to the dietary algal sterols. On the other hand, cholest-5-en-3 beta-ol is slightly depleted in C-13 by up to 1.7 parts per thousand and significantly enriched in D by up to 49 parts per thousand relative to the other shellfish sterols. This may suggest that shellfish sterols are derived from dietary algal sterols with no substantial isotopic fractionation during either assimilation or modification, but cholest-5-en-3 beta-ol is derived from an admixture of processes including de novo biosynthesis in shellfish. (c) 2005 Elsevier Ltd. All rights reserved.